Techniques for fabricating optoelectronic components make use of a dry etching technique such as reactive ion etching.
Such techniques require in situ monitoring of the depth of etching into the structure constituting the component.
It is common practice to use laser interferometry to perform such monitoring when the structure is made up of thin layers.
Laser interferometry makes it possible to take measurements by means of a reflected laser beam while etching is taking place. The received response signal as reflected from the structure is a function-of the rate of etching and of the complexity of the refractive indices of said structure.
During the etching of a structure made up of a various layers of material, each interface gives rise to a discontinuity in the reflected signal. The derivative of the signal (in the mathematical sense of the term) shows up as a peak that can be used to detect a particular interface.
This property is used to monitor accurately the depth of etching by inserting a marker layer in the structure where growth is to be stopped. The controlling operator or computer can then detect the interface and stop the etching treatment.
The method described above suffers from difficulties of implementation since it becomes uncertain when interfering peaks appear in the response signal or when the structure of the component presents multiple interfaces.
To remedy the problem of interfering peaks appearing, certain interferometer systems installed on dry etching equipment are designed to open a time window for detection and analysis purposes, thereby making it possible to ignore all interfering peaks that lie outside said window.
Unfortunately, that solution requires the rate of etching and the thickness of the material that is to be etched to be known so as to be able to open the detection and analysis window for the reflected signal when the depth of etching has come close to the position of the interface.
Furthermore, that method is not reliable since it does not make it possible to solve the problem of random peaks appearing.